The present invention relates to a method for automatic control, on the basis of the reception frequency, of the frequency of a radio telephone. In this method the reception frequency and the output frequency of a phase-locked loop are mixed in a first mixer; the output frequency and the frequency obtained from a local oscillator are mixed in a second mixer; and the frequency of the phase-locked loop is controlled on the basis of the frequency deviation of the output frequency of the second mixer. The invention also relates to a circuit for carrying out this method.
Previously, analog methods have been used in which a control voltage proportional to the frequency is formed. This control voltage regulates the frequency of the local oscillator. There have also been implemented methods directed to the object of the present invention which operate according to the counter principle. In these methods, a fixed reference oscillator is used, for example, a crystal oscillator of a processor as disclosed in patent application FI 864 909.
The disadvantages of the analog methods include the temperature creep appearing in them, deterioration of the circuits as they age, and the need for calibration, i.e. tuning of the circuitry, at the manufacturing stage. In known apparatus using a fixed reference oscillator and operating according to the counter principle, at least two separate oscillators are needed; one of which, the reference oscillator, must be very precise and stable. This is costly and requires complicated circuitry.